Welcome to Aerospace
Engineering

You are now at the first screen
of the Design-Centered Introduction (DCI) to the Aerospace Digital Library.For fast access to subject areas beyond the
Introduction course, please use Table 1, below. For guided access, skip
Table 1 and proceed with the introduction course.

Table 1: Direct Access
to the Sub-Disciplines of Aerospace Engineering

For over a century, aerospace engineers have
led the progress of human technology, and brought the world closer together.
Most simply, aerospace engineering is the realization of grand dreams through
careful scientific thinking and planning, bold but informed innovation,
and dedicated pursuit of perfection. It is the broadest of
engineering disciplines, because it takes the best of all human knowledge
to design, build, sell and operate a new (and always better!) aircraft
or spacecraft, and to use it to the best advantage. Many aerospace projects
appear so "far-out" that most people dismiss them as impossible, until
they actually see them working: it is up to the AE to figure out these
dreams, and reduce them to simple, step-by-step designs which are clean,
simple, safe, cheap and reliable, so commonplace that anyone can use them
and feel at home.

So don't be surprised when you read
that you can learn to design an airliner, starting out with a high-school
background. The approach we take in this course is called the "Runway across
Canyons".

The various disciplines of aerospace engineering, such as aerodynamics,
propulsion, etc. are like mountain ranges. Sometimes we feel like we have
to climb down into a canyon and then up a steep wall to get to another
discipline, i.e., to really understand all the things that people have
figured out over the years. In this course, we lay out a "runway", bridging
these canyons, so that we can go at high speed from aerodynamics to propulsion
to flight mechanics, etc., on our way to developing our own conceptual
design for an aircraft.We do have a few resources, shown on the
control panel of our craft, as we start the takeoff roll...

In this course we will use the motivation of designing a specific vehicle
to learn about the various areas of aerospace engineering. So we will go
off into one area after another, but always come back at the end of that
detour, and do some more calculations or refinement of our design. All
that you need is a notebook and pencil, a calculator for elementary calculations,
and a spreadsheet.

2. Some Dreams of Today

The above picture is from the Clip Art provided with Deneba
Canvas 5 software.(Question: Which is the front end of this contraption?)The Wright Flyer must have looked incredibly sophisticated to the people
of 1903. And it was. It was a canard design, with wings which had variable
camber and twist, and the pilots had to perform extremely well. It could
take off from an unprepared runway, under gusting wind conditions, and
fly nap-of-the earth (very low altitude), an operation calling for precise
flight control. Fortunately the beach at Kitty Hawk was quite flat.

Likewise, today's designs look extremely sophisticated to us. They can
fly over 100 times as fast as the Wright Flyer, and go right out into Space,
circle the earth every hour or so, and return to precise touchdowns on
earth. Have we reached the limits of aerospace engineering? Many people,
even in the 1920s, thought that airplanes had reached the limits of speed
and altitude, and had detailed theories proving that not much more could
be gained by investing in thought or development of these wierd machines.
And today, still, we are just beginning. We have only about 100 years of
powered flight experience, whereas the birds and insects that we see have
evolved through maybe a million years of experience. We can't yet match
them for control precision, landing versatility, payload fraction, engine
weight fraction, fuel costs, maneuverability, reconfigurable geometry,
or structure weight fraction. Our machines are fragile and clumsy:
if their engines quit or a piece breaks off, they fall down quickly
or even catch fire. They have stiff, rigid wings that can't flap, twist,
fold or thrust to any significant degree. They need long runways and complex
traffic control systems. You have to drive through 2 hours of downtown
traffic and spend an hour and a half at the airport and another 30 minutes
on the taxiway to make a flight of 200 miles. When we launch spacecraft,
only about 30% of the structure and 10% of the total launch mass ever reaches
orbit: the rest is wasted.

The picture is of a rumored "Aurora" aircraft which is rumored
to be in flight testing from super-secret Air Force Bases. Some years ago,
when the F-117 was still super-secret, there were plastic hobby kits of
the F-117 available, and they had elegant shapes like that above. When
unveiled, the F-117 did not look much like those shapes.

Here are some dreams to consider: some are a lot closer than the others.
In each case, try writing out a mission specification, and a typical mission
profile, and then maybe you'll keep going, and figure out the detailed
design. Someone will, sooner or later, and most of these things will get
much closer to reality within the careers of today's students. Consider
that when today's professors were born, no human had ever reached orbit
(well, excluding anyone kidnapped by green-costumed visitors from the Andromeda
Galaxy..)

There are many kinds of flying vehicles today:
helicopters, balloons, fixed-wing aircraft (the X-29 is shown), and the
Space Shuttle are examples of designs which look drastically different
from each other, and are designed for very different missions.

National Aerospace Plane (NASP) hypersonic airbreathing vehicle concept
from NASA, presented along with President Reagan's call for "the Orient
Express", a vehicle which can fly across the Pacific Ocean in less
time than it takes to get from Atlanta suburbs to the Atlanta airport.
The Space Shuttle is certainly a hypersonic vehicle, except that it takes
a whole army to get it ready for each flight, and several weeks to turn
it around for the next flight, and it uses rocket propulsion, where all
the fuel and the "working fluid" has to be carried on-board from ground
level. Picking up the oxygen-laden air en-route should make hypersonic
flight much cheaper, if this can be figured out completely: this is called
"airbreathing propulsion". One difficulty with a hypersonic passenger
craft is that the acceleration and deceleration phases would be quite "interesting"
for most passengers if one flies a direct route. This can be made
less stressful by going around the earth once, which would add another
2 hours or so to the flight time, but would require going to an even higher
speed and altitude. It might also raise the expectations of the passengers
with respect to the food service (the direct route will have a very short
cruise segment, which only merits peanuts/pretzels by today's standards).
Another interesting statistic (Aerospace America, Oct. 1998) is that roughly
75% of astronauts, who are all superbly fit and trained professionals,
get various symptoms of motion sickness during space missions, despite
medical precautions. So it is likely that the initial hypersonic "airbreathing"
vehicles to be revealed will in fact be (or already are) missiles, uninhabited
bombers, and perhaps later, some missions flown by military pilots. The
"Orient Express" that President Reagan described is still a few years away,
and will probably be replaced by a High Speed Civil transport flying at
lower supersonic Mach numbers (1.7 to 3.5).Yet another interesting
issue (one of very many) is that the surface temperatures generated during
high-speed flight might make it difficult to open the doors for some extended
duration after landing, so people might get very tired standing up in the
aisles with their hand-baggage after the "fasten seat-belt" light goes
out at the airport gate.

Of course these are not unprecedented problems: flight on the venerable
DC-3 Dakota airliner , which was the best option available to many of us
when we were younger, also used to make many people sick from the continuous
buffeting, and caused piercing ear-aches, partly from the pressure changes,
and partly from the pleasure of sitting for hours close to something
that sounded like five diesel locomotives at full power.

Use this link to see how the various subject areas come into the design
of an aircraft.

4. (a) Mission Specification

Where does one start, to go about designing
one of these grand contraptions? The answer is quite easy when one
stops to think about it. First, we have to decide what we want the
contraption to do. We will write out a wish-list, then think about
it and perhaps constrain those wishes just a little. Then we will think
of what a "typical mission profile" might be. For example, we will consider
the design of a large airliner, one which is slightly bigger and faster
and can go further in greater comfort, and cheaper, than the best of today's
airliners. Our aircraft is to carry 400 passengers, non-stop, 10,000 miles,
and do this with the comfort-level of today's Business Class for everyone.
As we write out the mission profile, various other requirements occur to
us. The aircraft must be able to take off from any large city airport,
in hot weather. Such as Denver or Mexico City (5000+ feet above sea-level),
where the temperature may be 100 deg. F and very humid (well, at least
in Mexico), and still fly the full range and payload. And be able to take
off, no problem, even if one engine quits just as the aircraft is lifting
off the runway. And land safely even if one engine quits when the aircraft
is as far as it can be from any airport. And have enough fuel left at the
destination to be able to fly another 500 miles, or loiter for 1 hour,
because the weather may be bad at the destination... And....And...
The list gets much, much longer as we think about the detailed design,
later. Aerospace engineers think about everything that can possibly go
wrong, and many things beyond that. And then they worry, and plan, and
check their calculations, and talk to other people about how to improve
their estimates and calculation procedures. They develop simulators to
test out every eventuality. Nothing is left to chance. And yet, we know
that things still go terribly wrong sometimes, so there's always more to
think about...

Table
1: Simplified Design sequence

Step

Issues

Define
the mission

What
must the vehicle do?

Survey
past designs

What
has been shown to be possible? (don't worry about WHY yet)

Weight
estimation

How
much will it weigh, approximately?

Aerodynamics

Wing
size, speed, altitude, drag

Propulsion
and engine selection

How
much thrust or power is needed? How many engines? How heavy? How much fuel
will they consume?

4
(b) Weight Estimation

One simple way to start the
conceptual design is to realize that we are designing something that must
lift some weight and carry it a certain distance. The mass to be carried
is the "payload": the load which we (hopefully) get paid to carry. Once
the payload is determined (as simple as figuring out how much the passengers,
their bags, food, etc. will weigh), we ask: "Haven't others tried to do
something similar or close to this? How much did their aircraft weigh?
We know we are smarter than anyone else, but maybe they too thought carefully,
and maybe we can learn something from the results that they got". This
is called "benchmarking". From this, we can get a rough idea of the weight
fractions of the various systems involved. For example, it is a rough "rule
of thumb" that the fuel weight may be as high as 50% of the take-off weight
of a large airliner which is to fly a very long distance. This applies
also to birds flying across oceans (Ref: Tennekes): they eat until they
can barely get off the ground even with a long takeoff run on the beach,
running into the wind to increase airspeed.

Table
3: How the Take-off Gross Weight (TOW) of an Aircraft is broken out among
the systems

Now, if we can figure out the
payload weight (which only we know, based on the intended mission specification),
and we can get the payload fraction from somewhere, (maybe by taking an
average of existing designs and being slightly more ambitious) the Takeoff
Gross Weight is simply the Payload divided by the Payload fraction.

For
example, if the Payload is 30,000lbs, and the Payload fraction is 0.15,
then the TOW is 30,000 / 0.15 = 200,000 lbs.

This
is of course an estimate. The rest of the design is to make sure we come
in under this estimate, when we calculate everything else. When we have
a rough calculation of all the other things, we'll go back and "iterate",
many times: refine our estimates, so that the whole vehicle gets better
and better.

4
(c) Benchmarking

Hunting through the available
data on various aircraft, we find that there is a wide range of answers
to our question on the payload fraction. Some craft weigh only 5 times
their payload; others weigh 90 times the payload. As we look closer, we
see that there is some similarity between these "payload fractions"
for aircraft which have similar "missions" and payloads. So in our case,
we ignore the fighter designs and the space launcher designs and the helicopters,
and the birds, and focus on large airliners, like the Boeing 747, 767,
777, Airbus A340, A320, McDonnell-Douglas MD11, MD90, and the Lockheed
L-1011. These are all aircraft meant to carry large numbers of passengers
(100 to 500) over long distances (upto 8000 miles). We find, however, that
no one has quite designed an aircraft which can do all the things that
we want our aircraft to do. So we are on our own in that respect, going
out into the unknown.